<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<style>
#one{
font-size:18px;
}
#two{
font-size:24px;
}
#three{
font-size:36px;
}
</style>
<body>
<p id="one">ID选择器</p>
<p id="two">ID选择器2</p>
<p id="three">ID选择器3</p>
</body>
</html>